1. Field of the Invention
The present invention relates generally to a type of fuel cell system and a method of operating the fuel cell system.
2. Description of the Related Art
In a solid-state polymer type fuel cell, an anode catalyst electrode layer and a cathode catalyst electrode layer are respectively arranged on the two surfaces of a solid-state polymer electrolyte membrane having hydrogen ion conductivity to form a membrane electrode assembly (MEA). Then, when a fuel gas containing hydrogen is fed to the anode catalyst electrode layer while air containing oxygen is fed to the cathode catalyst electrode layer, the following electrochemical reactions take place:[Anode]2H2→4H++4e−  (Equation 1)[Cathode]O2+4H++4e−→2H2O  (Equation 2)
Good hydrogen ion conductivity is exhibited in the solid-state polymer electrolyte membrane when the membrane contains water. Consequently, during operation of a fuel cell system, it is necessary to humidify the solid-state polymer electrolyte membrane appropriately. Also, water is generated in the electrochemical reaction shown in Equation 2 above at the cathode.
When said solid-state polymer type fuel cell is in a shutdown state, the interior of the fuel cell should have a water content most appropriate for restart. In particular, a technology described in Japanese Kokai Patent Application No. 2005-209634 has been developed for fuel cell systems to be used in environments below the freezing point wherein the water contained may freeze and clog the gas flow paths, and so on. According to this technology, when the fuel cell system is shut down, a purge should be performed so that the residual water content is optimum to ensure that residual water will not clog the flow paths, and that the resistance of the electrolyte membrane will not become excessively high. In addition, with regard to the method for judging the optimum residual water content condition, a determination may be made that the optimum residual water content condition is met when the gas pressure differential between the inlet and outlet of the reaction gas flow path is below a prescribed level, and the membrane resistance of the electrolyte membrane exceeds a prescribed level.
In the aforementioned methods, however, the water/air purge process performed before the system is shut down is finished very quickly. Consequently, there is little change in the electrolyte membrane resistance. As a result, it is difficult to estimate the amount of residual water left in the fuel cell stack based on the resistance of the electrolyte membrane, which can be problematic.